Metallocene catalyst systems are extensively used in a variety of polymerisation systems, including the polymerisation of olefins. Generally, in order to obtain the highest activity from metallocene catalysts, it has been necessary to use them with an organoaluminoxane activating agent, such as methylaluminoxane (MAO). This resulting catalyst system is generally referred to as a homogenous catalyst system since at least part of the metallocene or the organoaluminoxane is in solution in the polymerisation media. These homogenous catalyst systems have the disadvantage that when they are used under slurry polymerisation conditions, they produce polymers which stick to the reactor walls during the polymerisation process (generally referred to as “fouling”) and/or polymers having small particle size and low bulk density which limit their commercial utility.
Various methods have been proposed in an effort to overcome the disadvantages of the homogenous metallocene catalyst systems. Typically, these procedures have involved the prepolymerisation of the metallocene aluminoxane catalyst system and/or supporting the catalyst system components on a porous carrier (also known as a “particulate solid” or “support”). The porous carrier is usually a silica-containing support.
Another important consideration in the development of metallocene catalysts is the yield of solid polymer that is obtained by employing a given quantity of catalyst in a given amount of time. This is known as the “activity” of the catalyst. There is an ongoing search for metallocene catalysts and techniques for preparing such catalysts which give improved activity for the polymerisation of olefins. An improved activity means that less catalyst needs to be used to polymerise more olefins, thereby reducing the costs considerably, since metallocenes are more expensive than Ziegler-Natta and chromium catalysts.
Several attempts have been made to titanate silica supports for use in metallocene catalysed ethylene polymerisations. Jongsomjit et al. (Molecules 2005, 10, 672, Ind. Eng. Chem. Res. 2005, 44, 9059 and Catalysis Letters Vol. 100, Nos. 3-4, April 2005) discloses the titanation of silicas for zirconocene catalysed ethylene polymerisation, wherein the support is prepared according to Conway et al. (J. Chem. Soc., Faraday Trans. J, 1989, 85(1), 71-78) using mixed supports of titania and silica mixed-oxide supports. The increase in activity with such a support is only of 25%. Under polymerisation conditions, little morphological control can be obtained with such a support. It is particularly difficult to use industrially, since the porous volume, bulk density and particle size of both the silica and titania need to be similar in order to avoid decantation of one with respect to the other. In addition, the interaction of the Ti with the actives sites is not optimized. Furthermore, the zirconocene (metallocene) catalyst is not incorporated into the mixed-oxide support, but added separately into the polymerisation reactor in the presence of 1-hexene, thus during polymerisation.
Fisch et al. discloses immobilization of metallocene within silica-titania by a non-hydrolytic sol-gel method (Applied Catalysis A: General 354 (2009) 88-101). However, the MAO is used as a cocatalyst during the polymerisation process. The MAO and the aluminum thereof does not form an integral part of the support, thereby allowing the formation of large amounts of undesirable TiOH and SiOH on the co-gel.
U.S. Pat. No. 6,395,666 B1 discloses a catalyst composition that comprises an organometallic compound, an organoaluminum compound, and a fluoride solid oxide compound. No alumoxane is added to said composition.
US 2003/0232716 A1 discloses a catalyst composition that comprises an organometal compound, an organoaluminium compound and a treated solid oxide compound. No alumoxane is added to said composition.
U.S. Pat. No. 5,604,170 discloses the use of titanium compounds, in particular titanium tetrachloride, however not in combination with metallocene and alumoxane. None of the disclosed Solid Catalyst Components of the examples show a titania-silica co-precipitated support comprising an alumoxane and a metallocene.
U.S. Pat. No. 5,124,418 discloses silica, alumina or silica-alumina inorganic oxide support that may be employed in combination with magnesia, titania, zirconia and the like. Alumoxanes are not incorporated into the support, but are added separately into the reactor. There is no mention of trying to improve catalyst activities using the presence of titanium.
EP 0 514 594 A1 discloses a catalyst precursor composition supported on a porous carrier comprising a magnesium compound, a zirconium compound and a titanium or vanadium compound. However, the silica and titanium compound are not co-precipitated together.
Thus, a new catalyst support is needed for metallocene catalysts which can induce improved activity of the metallocene catalyst system, particularly under industrial conditions.
An object of the present invention is to provide a new catalyst support for metallocene catalysts to increase their activity.
Furthermore, it is an object of the present invention to provide a new method for polymerising olefins, preferably ethylene and propylene, using a new supported metallocene catalyst system.